Bushings are devices that are usually used to carry current at high potential through a grounded barrier, e.g., a transformer tank. In order to decrease and control the electric field near the bushing, condenser bushings have been developed, also known as (fine-) graded bushings. Condenser bushings facilitate electrical stress control through insertion of floating equalizer (electrode) plates, which are incorporated in the core of the bushing. The condenser core decreases the field gradient and distributes the field along the length of the insulator, which provides for low partial discharge readings well above nominal voltage readings.
The condenser core of a bushing is typically wound from kraft paper or creped kraft paper as a spacer. The equalization plates are constructed of either metallic (typically aluminium) inserts or non-metallic (ink, graphite paste) patches. These plates are located coaxially so as to achieve an optimal balance between external flashover and internal puncture strength. The paper spacer ensures a defined position of the electrodes plates and provides for mechanical stability.
The condenser cores of today's bushings are impregnated either with oil (OIP, oil impregnated paper) or with resin (RIP, resin impregnated paper). RIP bushings have the advantage that they are dry (oil free) bushings. The core of an RIP bushing is wound from paper, with the electrode plates being inserted in appropriate places between neighbouring paper windings. The resin is then introduced during a heating and vacuum process of the core.
A disadvantage of impregnated paper bushings is that the process of impregnating the pre-wound stack of paper and metal films with oil or with a resin is a slow process. It would be desirable to be able to accelerate the production of high voltage bushings, which bushings nevertheless should be void-free and safe in operation.
The document DE 19 26 097 discloses a high-voltage bushing having a conductor and a core surrounding the conductor, wherein the core comprises spacers, which spacers are impregnated with an electrically insulating matrix material. The spacers have a multitude of holes that are fillable with the matrix material. Each spacer is formed from a mesh of electrically insulating glass fibers in form of a cylindrical tube. For each glass fiber tube, glass fibers are formed around a cylinder and they are impregnated with an epoxy glue and afterwards hardened. Then the hardened spacer tubes are (partially or fully) coated with a conductive (metallic or semiconducting) material, which constitute the equalization plates. The bushing comprises these spacers in form of tubes, which are arranged concentrically around the core. For the impregnation process, the spacer tubes have to be fixed in a mould in order to ensure their correct position and in order to avoid that neighbouring tubes touch each other. Then a particle-filled resin, which is used as a matrix material, is filled into the mould. As several glass fiber tubes of different diameter have to be produced for the production of each bushing and as these tubes have to be put into each other with their position fixed, this method for production is rather time consuming. Besides, for each type of bushing a specific mould has to be made.
GB 690 022 describes an insulator made of spirally wound paper. Paper layers with lines of conductive or semi-conductive material, which are spaced apart from one another, are wound together with unlined paper in order to achieve a spirally wound bushing, which is then impregnated with an insulating liquid, such as oil.